Isolation and pathogenicity of some South African entomopathogenic fungi (Ascomycota) against eggs and larvae of Cydia pomonella (Lepidoptera: Tortricidae)

Abstract

Virulent entomopathogenic fungal strains against Cydia pomonella (Lepidoptera: Tortricidae) were isolated and identified in the Western Cape Province of South Africa. Thirty-nine isolates belonging to six species were obtained using the insect bait method. Generally, Metarhizium robertsii (Ascomycota: Hypocreales) was the most frequently encountered species representing 51% of the total number of isolates collected from the soil samples. This is the first report of M. robertsii from southern Africa. Mortality data from an immersion bioassay indicated that the 39 fungal isolates were pathogenic against fifth instar larvae of C. pomonella inducing 47–85% insect mortalities. Two M. robertsii strains, MTL151 and GW461, induced 85% larval mortality and were selected for further evaluations. The exposure of freshly laid eggs to wax papers that were pre-treated with fungal spores ranging from 10³ to 10⁸ spores ml⁻¹ of MTL151 and GW461 resulted in a significant reduction of egg hatchabilities, from 95 to 66% and 93 to 71%, respectively as spore concentration increased. First instar larval neonates were exposed to apples that were pre-sprayed topically with varied conidia suspensions (10³−10⁸ spores ml⁻¹). The mean percentage of participating apples with larvae in their cores/flesh significantly reduced from 53 to 10% (MTL151) and 76 to 10% (GW461) of 10 apples, and a concurrent decrease in the incidence of apple fruit rot was observed as conidia concentration increased. Up to 90% of apples treated with 1 × 10⁸ spores ml⁻¹ had no larvae present in their cores and this result compared favourably with the commercial pesticide Fruitfly®, containing the active ingredient cypermethrin (20 g/l) used at a recommended dose of 0.25 ml/250 ml of water.

Keywords:

• entomopathogenic fungi
• Metarhizium robertsii
• Cydia pomonella
• anti-egg hatchability
• biological control
• apple fruit protection

Acknowledgements

The authors thank Mr. M. Wohlfarter (Entomon Technologies [Pty] Ltd.) for supplying the different life stages of codling moths, Dr T.L. Bloemfield (Agricultural Research Council, South Africa) for technical advice on codling moth and Prof. K. Jacobs (Stellenbosch University) for assisting with fungal identification. We thank Dr N.K. Maniania (ICIPE) and Dr P. Addison (Stellenbosch University) for their constructive comments and suggestions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was funded by Cape Peninsula University of Technology through the University Research [grant number R166].